The invention relates to method for measuring oxygen and/or the air/fuel ratio lambda and nitrogen oxides in gas mixture. The sensor has a reference electrode representing a constant oxygen partial pressure, at least two electrode pairs, wherein two electrode pairs each have a common electrode associated therewith The gas sensors also have solid electrolytes arranged between the two electrodes of each electrode pair directly at the electrodes and electrical leads for connection and for take-away of electrical measurement signals.
A gas sensor of a this type is known, e.g., from German published patent application DE 195 34 918 A1. The sensor therein has two electrodes constructed as mutually engaging comb structures (see FIG. 1), which are arranged on the side of the solid electrolyte facing he gas being measured (hereinafter "measurement gas"), and a reference electrode is provided opposite thereto on the reference air side. That invention is directed mainly to a reliable seal to ensure that no effects are exerted on the operation and performance of the two electrodes (sensor contacts) provided on the measurement gas side of the solid electrolyte. This construction makes possible a voltammetric measurement of two gas components in a gas mixture.
In addition, a gas sensor of the generic type is known from German published patent application DE 36 10 366 A1, in which a plurality of electrochemical measuring cells are arranged on a tubular support This device allows only gaseous pollutants to be measured (not oxygen). The evaluation of the measurement signals takes place based on the characteristics of the pollutant concentrations.
Furthermore, a gas sensor of this type is known from German Patent DE 41 09 516 C2. In this device, the solid electrolyte is constructed in the shape of a platelet, on one side of which an electrode is applied which functions as a reference electrode, and on the opposite side of which at least two measuring electrodes are applied, which interact with various components of a gas mixture. The platelet-shaped sensor is built into a housing, which is then to be installed as a gas probe in the exhaust gas duct of a motor vehicle, more specifically perpendicular to the flow direction of the exhaust gas.
This probe functions without a reference gas, which is required for obtaining an electrode potential independent of the environment. However, such electrodes are not stable with respect to their electrochemical potential, especially when the mixture composition changes from lean to rich. In addition, with configurations of the sensor design which are not rotationally symmetric, it is very hard to realize a stable and uniform temperature distribution over the entire surface.
The nitrogen oxide (NO.sub.x) measuring electrode and the negative pumping electrode are two electrodes, which are arranged locally separated from each other. It is difficult to establish a low oxygen concentration at the nitrogen oxide measuring electrode, since more oxygen is always present which cannot be removed by the pumping electrode, because it does not reach this pumping electrode.
Similar gas sensors, relatively complicated in construction, are also known from German published patent application DE 42 25 775 A1. In the sensors described there, a constant voltage is applied respectively between a reference electrode and a measuring electrode, as well as between the measuring electrode and a counter-electrode, and the current exiting from the counter-electrode is measured as a function of the nitrogen oxide concentration.